Method for providing a tubular node in a framework truss structure such as offshore platforms for oil drilling and production

ABSTRACT

A method for providing a tubular node in framework truss structures, such as offshore platforms for oil drilling and production activities of the type consisting of interconnected legs and braces of steel tube elements. For formation of the transition between leg and brace in the node is utilized a blank of rolled plate steel, which is given the desired curved shape corresponding to the curvature of the leg. The blank is pressed or extruded in a suitable press tool formed with an outwardly extending brace stub at the desired angle, which brace stub is designed for welding to a complementary brace tubing in the framework. Thereafter, the blank with the brace stub is welded into the leg proper and constitutes a wall segment of the leg in the node.

The present invention relates to a new solution for construction oftubular nodes in framework truss structures of the kind consisting ofinterconnected legs and braces made of steel tubes. Such structures arebeing used to large extent in stationary and mobile offshore platformsfor oil drilling and production activities.

In conventional construction approaches for such structure the bracesare directly welded to the legs to form the tubular nodes, i.e. thejoints between the legs and bracings. Such nodes may consist of a singlebrace and leg or of multiple bracings joining the leg in one or multiplanes. Although direct welding of the braces to the leg is a simplesolution from point of view construction, it is fully realized that itcauses many serious problems and detriments. Firstly, it requirescareful and costly preparation and adaption of the surfaces to beconnected by welding, particularly if the braces are joined in anoblique angle. The welding is also complicated by the fact that the wallthickness of the connected tubes is, as a rule, different. Further, thesubsequent control of the weld quality, for instance by means of ultrasound or X-rays of such welds is often very complicated because oflimited accessibility. Experience has shown that because of this, oneoften can not ensure the quality of the welding work. These problems areamplified, in the case of angular and multibrace nodal joints.

In order to increase the stiffness and strength of the nodes it is oftennecessary to increase locally the wall thickness of the leg at the nodeor also the wall thickness of the braces. Such measures contributehowever further to the welding problems and lead also to increasedweights and higher costs of the platform.

Also from structural-strength point of view, the conventional solutionis far from ideal. The junctions brace/legs represent geometricaldiscontinuities which give rise to very high material stress peaks andstress concentrations at these locations. The fact that in traditionalsolutions the connecting weld is placed at these very locations ofextremely high stress concentration will necessarily result in a veryserious further detriment to the strength and safety of the node. Thisbecause the environment wave loads are dynamic in theis nature, and thelatter together with the high stress concentrations at the weld toes ofconventional nodes give rise to a severe fatigue strength problem whichmay result in development of early cracks at the weld toes andsubsequently failure of the connection arising from these locations.

Serious failures in platform rig structures during the past years,especially with those used in severe environments such as the North Sea,have proven that in the design of such offshore platforms both thestatic and the fatigue strength of the structure must be thoroughlyconsidered and its adequacy must be satisfactory verified. This impliesfirst of all that the node and the members between the nodes mustpossess sufficient strength. This implies that the structures shouldshow sufficiently low probability of failure with respect to thefollowing failure modes:

1. Static strength of the node, i.e. a stability to sustain extremeloads without undue permanent deformation, collapse, or even crunchingshear rupture.

2. Buckling/yielding of the members between the nodes.

3. Fatigue failure due to crack initiation and crack propagation.

In order to create a stronger and more durable joint between legs andbracings in frame work structures of this kind it has been proposed toavoid the welding at the junction of the tubulars where very high stressconcentration occur and instead to form brace stubs in one-piece nodewhich is prefacricated and is welded in place as section of thestructure directly to the legs and bracings. Such solutions have,however, hitherto not been adapted by the platform rig industry and hasas far as is known not been tried out in practice. The main objectionhas been that in connection with casting or moulding there will alwaysbe present an inevitable risk for moulding defect such as internalbladders and cracks, and such possible defects cannot be disclosed withabsolute 100% warranty in connection with known control equipment.Furthermore will the casting of cast work of this size fall undueexpensive and time consuming, and the casting must usually take place ina factory located at a remote distance from the building site of therig.

A number of similar solutions in order to provide improved nodes forframework for platform rig constructions have been proposed. As examplesof other solutions can be referred to Norwegian patent specificationsNo. 140 949, 142 454 and No. 145 731. These patents show differentembodiments for nodes, made in relatively small dimensions and as solidpieces of goods made of die forged or cast steel, in some casesreinforced with welded cross pieces, etc. The node bodies areprefabricated with the desired stubs to be welded to the bracing in theframework structure. It is further as a supplement to these solutionsproposed to use special conical transition pieces between the node andthe pipe bracing having larger dimensions such that the nodes can bemade with the prescribed small overall dimensions. Small dimensions inthe node are obviously the ideal from a static design point of view andthey are otherwise assumed to be a possible solution, from aconstructional point of view, partly because one can utilize so-calledisotropic material, i.e. material which has the same strength propertiesin all directions (in contrary to rolled plate goods which normally areweaker in transverse direction), partly because small dimensions resultin small moments of inertia and thereby reduced bending stresses.

In spite of their apparent good merits--both in regard design and inregard static forces--such solid node solutions have, however, not beenaccepted in connection with larger platform rig constructions. Onereason for this situation may be that the nodes fall expensive and timeconsuming to make, frequently at a production site which is remotelylocated from the building site of the rig. An important practicaldrawback with such nodes is that the legs cannot be utilized as adownlead for piles or passage for drilling equipment, productionequipment etc.

A further proposal for node solution consists in that the leg isencircled by a collar-like ring which is provided with one or moretransition pieces designed to be joined to the bracing in the framework. In a preferred embodiment for such solution is utilized a specialcollar with transition pieces to each separate bracing in the nodes. Asuch collar solution can in many ways seem more attractive than theprecedingly described solid mass node solution, but the solution giveson background of the large or extended dimensions of the node itself asource for large stress concentrations and also considerable adaptationproblems, particularly in the transition pieces on the collar.

In general one can conclude that in connection with large frame workstructures for purposes such as offshore platform rig constructions, thenodes constitute the weakest link in the structure, both in regard thetotal strength of the rig and in regard the lifetime of the rig, due tothe prevailing risks for fatigue failures in the nodes.

The main object of the present invention has been to develop a newimproved node solution for large frame work structures made of steeltubing.

A special object of the invention is to provide a method for makingnodes adapted to be joined with bracings extending both at a right anglerelative to the leg and at tilted or oblique angles, such as 45°relative to the leg in direction upwards or downwards.

The method in accordance with the invention has the aim to provide atubular node in frame work truss structures, such as offshore drillingplatforms and the like designed for severe wave conditions, andconsisting of legs and braces of steel tube elements, and the inventionis characterized in that for the formation of the transition between theleg and the bracing is utilized a transition member made from a plateblank of rolled steel, which plate blank is given the desired curvedshape corresponding to the curvature of the leg in that said blank, andby means of pressing or extrusion in a suitable press tool, is beingformed with one at desired angle extending brace stub designed forwelding to a complementary brace tubing in the framework, whereafter theplate blank stub is welded into the leg proper and thereby constitutinga wall segment thereof at the node.

In dependence upon the size and other criteria of the node structure,the pipe stub formed can present a mouth opening in a plane positionednormal to the stub axis or at an angle thereto, for instance in a planebeing parallel with the leg axis.

The forming of the plate element including the pipe stub can take placein one separate press and/or extrusion operation or in severaloperations with or without repeated, intermediate heat treatment inorder to recover the ductility of the material.

The special tool to be used to make the stub in the plate element can becylindric or slightly conical, preferably with a rounded off orsubstantially spherical front end. The matrix is provided with acomplementary opening provided with a suitably rounded off transitionzone between the inside plane surface and the inside surface proper ofthe stub to be made.

A special advantage with the method is that trials have provided that itcan be used for the making of oblique stubs in the plate element. Thisis provided by using a tool with a piston or the like which is directedin the desired angle relative to the goods in the leg, and the matrix islikewise provided with an oblique opening given the correct, oval,rounded off opening zone.

The plate section including the stub can constitute a complete integralring section of the leg or only a wall segment of the leg. The latteralternative will be the only alternative in connection with legs havinglarge diameter. Such large legs will along the circumference consist offor instance five steel plate sections welded together as segments inthe annular structure. The plate section or segment which shall serve asa blank for the stub may have the same thickness as the other sectionsin the leg in the node part of the leg, but may alternatively have alarger thickness, either only in the plate section for the stub, or allthe plate sections in this circumferential or annular section of the legmay have an increased thickness.

Through the solution in accordance with the invention is provided animproved node design implying a number of other advantages in comparisonwith known technique, particularly versus the conventional solution withbracing welded directly onto the leg structure. Thus, one avoids sharpangles between the two parts which shall be joined together, whichotherwise inevitably lead to large stress concentrations. The goods inthe stub is given a uniformly reduced thickness from the thickness inthe adjacent leg section to the thickness in the complementary bracingpipe which shall be welded to the mouth opening of the stub. One avoidswelding joints just in these areas where the stress concentrations willbe most critical, namely the angular transition zone between the leg andthe bracing. By extruding or pressing a stub directly in or from the legmaterial, one will obtain a stub with goods which is somewhatanisotropic, i.e. a material which will have greater strength in thelongitudinal direction of the stub than in transverse direction. It hasbeen shown that this is a great advantage on background of the differentforms for stresses and strains which will be prevailing in the stub andin the node. The node must be designed with reference to static strengthi.e. its ability to sustain extreme brace loads without undue permanentdeformation, collaps, and shear rupture. This must be checked againsttensile loads as well as compressive forces. The latter, in the case ofunstiffened nodes, are as a rule more critical due to ovalization andcollapse mechanism of failures. Of other failure modes shall bementioned risk for buckling/yielding of the various members in the node,and finally fatigue failure due to crack initiation and crackpropagation.

However, calculations, practical experience and trials have shown thatthe most critical failure mode is failures due to fatigue especially inconnection with platform rig structures used in offshore environmentswith severe wave conditions, such as in the North-Sea, practical trialshave proven that the invention provides a solution which is especiallyadapted to overcome the last mentioned problems.

In addition to what is stated before a further reason for the excellentresults obtained with the invention can briefly be said to find itsorigin in that the critical compressive and tensile brace forces areuniformly transferred and distributed to a comparatively large area ofthe leg, and one avoids simultaneously abrupt changes in the goods andobtains instead an even curved or arched transition fillet between thebrace stub and the leg.

Furthermore, on background of the curved transitions between the stuband the goods in the leg, one will obtain access for positioningequipment for ultra sound and x-ray control of the entire transitionzone, simultaneously as the need for control will be reduced in thetransition zone. Furthermore, necessary control of the stub includingthe plate element including the stub can be made on the production sitefor the same, making it superfluous to move such equipment to theproduction site for the rig, and finally one obtains access to the jointfrom both sides, something which has not hitherto been possible inconnection with welded bracing where the leg surface will close off thejoint.

The advantages which therefore are obtained through the inventioncompared with known types of nodes may be summed up as follows:

1. The nodes will sustain higher fatigue loads.

2. The nodes will attain a much longer operation lifetime.

3. One avoids geometrically complex and costly tubular joints weldingworks.

4. The need for operational inspections of the nodes will be reduced.

5. Because of their great fatigue strength the nodes in accordance withthe invention increase the possibility to design slender and higher rigstructures for deeper water without making it necessary to use designcriterias other than for static loads (dimensioning for the 100 yearwave).

The invention also involves a number of other important features andadvantages which will appear from the following specification, whereinthe invention shall be described with reference to the accompanyingschematic drawings which illustrate some embodiments of the invention,and wherein:

FIGS. 1a and 1b show sections taken along horizontal and verticalsections, respectively, the latter in a large scale, of a node in aconventional truss or frame structure designed for an offshore platformrig and consisting of tube shaped legs and braces.

FIG. 2 is showing a plate or segment of a plate blank for a leg whichshall be provided with a brace stub, the blank as shown furnished with a"starting bore".

FIGS. 3a and 3b show the blank and the press or extrusion tool forformation of a angular brace stub and a straight brace stub,respectively.

FIG. 4a is showing the tool, i.e. both the piston and the matrix, duringthe extrusion operation, during the finalizing phase of the formation ofan angular brace stub.

FIG. 4b shows the blank and matrix for straight brace studs.

FIG. 4c is showing a fragmentary section shown in an enlarged scale.

FIG. 5 is showing a schematic horizontal section of a leg which is beingprovided with a plate segment including the brace stub in accordancewith the invention.

FIG. 6a is showing a schematic horizontal view through a leg with a nodeprovided with brace stubs in accordance with the invention.

FIGS. 6b-6e are showing vertical fragmentary use of a node provided withbrace stubs in accordance with the invention at various verticalelevational sections, figures b, c and d, the sections before assemblyand Figure e showing the completed node.

FIG. 7 is showing a node in accordance with the invention similar to thenode shown in FIG. 6e, but wherein the plate segment or sections can beselected arbitrarily,

FIG. 8 is showing altogether six versions of nodes in accordance withthe invention.

A conventional node as shown in FIG. 1 is made by forming the mouthopening of an adjacent bracing and brace stub with a correct arched orcurved opening edge the course of which will depend on the angle betweenthe brace and the leg and the same as positioned directly in contactagainst the leg surface. The welding connection or weld fillettherebetween is in this design normally limited to a usual triangular orV-shaped weld fillet along the external opening or joint between the legand the brace. In order to provide also an internal weld, the leg mayinitially be provided with a separate brace stub (frequently conical)which is welded on to the leg also with an internal weld whichthereafter must be controlled, whereafter the brace is welded on to thebrace stub with an annular weld, but then only with an external weld 7.With this design is created, particularly with angular bracing, aparticularly critical zone as shown in a circle designated with thenumber 8 in FIG. 1b. In this area the access for welding will be ratherpoor and the subsequent control by means of ultra sound or x-rayequipment will be similarly difficult, if possible at all. This israther unfortunate since just in this area the maximum or peak stresseswill occur. Such peak stresses can frequently exceed 10 and up to 30times the "normal stresses" in the node. The conventional solution witha weld in the node in this area will always imply a risk for fatiguefailure in the transition zone between leg and bracing, particularly asa result of repeated compression and tension stresses. The operationallife span of a such structure will be rather uncertain, particularly inconnection with offshore platform rigs in areas exposed to heavy waveactivities creating pulsing stresses of the beforementioned type.

FIGS. 3a, 3b and 4a, 4b and 4c illustrate the method for making a bracestub or transition piece to be used in a node in accordance with theinvention. A ready curved or rolled plane section or plate of rolledsteel as shown in FIG. 2 is provided with a starting bore 12. The plateblank is thereafter as shown in FIG. 3a or 3b positioned in a press- orextrusion tool having a matrix 14a (for angular brace stubs) or 14b (forstraight brace stubs). The matrix is shaped with a die surfacecorresponding to the curvature of the plate blank (not shown) and isprovided with an oval or circular opening 16, the dimensions of whichcorrespond to the outside dimensions or diameter of the brace stub whichshall be made. The opening or aperture in the matrix may in some caseswith advantage be contoured with a somewhat reducing diametric dimensionin direction outwards. When making an angular brace stub as shown inFIG. 3a the opening in the matrix is directed at an angle correspondingto the direction of the brace stub to be made, and the plate blank ispositioned and fixed (not shown), such that the starting bore 12 ispositioned somewhat disposed against the short side of the brace stub asshown. Thereby one can obtain that the blank goods under the extrusiondeformation is travelling or moving such that the final brace stubattains a substantially circular, transverse end opening, andfurthermore such that the goods gets a uniformly reducing thicknesstowards the end opening, but maintaining a substantially even thicknesscircumferentially, possibly with a somewhat larger thickness along theacute angular side in connection with angular brace stubs.

The press- or extrusion piston 20 as shown in FIGS. 3a and 3b and inFIG. 4a, is given a cylindric shape, possibly lightly conical indirection outwards, and the head or front end 22 is preferably given aspherical form alternatively slightly conical. The piston is supportedin a regular press chuck or support, such that the piston can be movedaxially towards the opening in the matrix. The piston should, however,also be arranged to undergo a certain lateral movement, since a lateralmovement is desirable in the making of angular brace stubs whereby thestarting bore is positioned somewhat laterally disposed relative to thestub axis, such as illustrated in FIG. 3a. In order to prevent initialfracture in the opening, the piston is initially positioned axiallystraight above the bore opening and is thereafter moved slowly towardsthe center axis corrolated with the gradual widening of the opening.

FIG. 4c shows the sectional fragmentary view in an enlarged scale of theencircled part shown in 4b and is illustrating the curvature 24 in thematrix opening 26, and a typical formation of a straight brace stub 28.

In the matrix opening may, if necessary, be positioned--in knownfashion--a dolly or counter tool 23 as indicated with stitched lines 22in FIG. 4a.

The press- or extrusion operation can take place in various fashionsdepending upon the product which shall be made and with or withoutpreheating of the steel plate blank.

If the extrusion operation shall take place without heating, theextrusion operation must be interrupted several times, i.e. theextrusion must take place stepwise. The number of steps will depend uponthe diameter of the brace stub and the thickness of the goods (usuallythe goods thickness will increase with the diameter of the brace stuband the brace). Between the extrusion steps one must carry out a heattreatment of the goods, such that the goods may pass through are-crystallization and thereby regain its ductility for furtherdeformation. One must continuously accurately control the extrusionoperation in order not to exceed the yielding point of the goods.

The press- or extrusion operation can alternatively be carried out witha preheating, for instance in the temperature range 800°-1000° C.

The extrusion operation can then in some cases be carried out in onesingle relatively long-lasting or long-moving step, or alternatively inseveral steps, sometimes with a renewed heating, such that the goods canregain its ductility.

Through the above described press- or extrusion operations is obtainedbrace stubs in goods which will have a socalled oriented structure, suchthat a high strength material is obtained, simultaneously as one obtainsan increased ductility in the extrusion direction. In a socalled coldpressing or forcing operation the blank will get rigid such that boththe yield point, the fracture point and the hardness will be greater inthe ready made brace stub than in the plate goods prior to thetreatment.

Practical trials have shown that one may in this fashion produce bracestubs of very high quality, and the stubs can have rather largedimensions, for instance 1 to 2 meters and having goods thicknesses inthe range up to 50-100 mm.

In making the tools it is obviously important that the opening edges inthe matrix are given a suitable rounded off curvature, since thiscurvature will define the bending radius of the brace stub in thecritical areas, i.e. the areas from the leg material proper to the stubproper.

Practical trials have shown that with a suitable design of the tool onecan reach optimal results, i.e. a gradually decreasing goods thicknessesin the stub from the thickness in the leg or wall thickness in thetransition piece which shall form a segment of the leg to the wallthickness in the adjacent bracing which usually shall have a far smallerthickness (usually 20-50% of the wall thickness in the leg). Practicaltrials have furthermore shown that one can reach the desired result thatthe thickness at the leg wall side is maintained along the criticalbending zone where the need for goods thicknesses is important. It willbe understood that through the invention one can entirely avoid weldingjoints in the areas or sections of the node where the critical or peakstresses will concentrate. In addition one will in these areas have ahomogeneous and in most cases stronger material than in the leg or thebracing proper.

In regard the length of the brace stub, this will for a stub extendingin 45° angle be of size substantially corresponding to the radius of thebrace, and not in any case below a length such that the goods at theacute angle side of the stub extends parallel or substantially parallelwith the center axis of the brace, such that one obtains a flushalignment along the transition to the bracing and preferably having acircular opening edge. In connection with straight stubs the length isnot particularly critical, but the stub ought to be at least so longthat the stub walls extend substantially parallel with the center axis,and further at least so long that the goods thickness along the openingedge substantially corresponds to the goods thicknesses of the adjacentbracing. One may, however, if desired, weld on to the stub a preferablyconical transition piece which can be given decreasing goods thicknessestowards the outer end or mouth for welding to the bracing.

To mention something about the actual strength of a node in accordancewith the invention, one can inform that the fatigue strength durabilityfor a node in accordance with the invention has through trials beenmeasured up to 10 times the lifetime of a conventional welded node ofsame type and same dimensions.

Alternatively one may on this basis find it permissible to increasecorrespondingly the allowable fatigue stresses and maintain the samelifetime as for a corresponding conventional welded node.

In some practical cases it may in connection with angular stubs,particularly if the angle exceeds 45°, prove difficult to obtainsufficient demands of goods in direction outwards on the longer side ofthe stub wall such that one can obtain the desired transfers end openingon the stub relative to the longitudinal axis of the stub withconservation of an even goods thickness around the circumference of thestub. In order to maintain an even goods thickness, particularly at theopening edge of the stub, something which is rather important, it may benecessary or desirable to make a stub which is shortened or slopedagainst the longer side of the stub wall. The mouth of the stub willthen be positioned in a plane which is not located transversely relativeto the longitudinal axis of the stub, and the mouth opening of the stubwill attain an oval or elliptic shape. Such shape will demand a matingelliptic shape on the mouth opening of the adjacent bracing which shallbe welded to the brace stub and complicates somewhat the joiningoperation between the node and the bracing.

As previously mentioned, FIG. 6a shows a schematic sectional plan viewthrough a node in accordance with the invention.

FIGS. 6b, c and d are showing three sections of a node in accordancewith the invention before assembly, and FIG. 6e is showing the sectionsillustrated in Figures b, c and d in assembled position, comprisingaltogether three brace stubs, each located in a separate vertical spacepart of the leg. The leg portions of the node are such designed that thecenter axes of all brace stubs coincide at the vertical center axis ofthe leg thereby obtaining a statically stable design.

FIG. 7 is showing a node structure similar to the one shown in FIG. 6e,but without defining any special separation lines in the leg structureforming the node indicated that the plate segments including the bracestubs can be selected arbitrarily in accordance with specific demands.

The above described and shown examples are only shown with the aim toillustrate the method in accordance with the invention. The scope of theinvention is defined by the attached patent claims.

We claim:
 1. Method for providing transition member between structuralelements for a node in a frame work truss structure, particularly foroffshore platforms such as drilling and production platforms, saidmethod comprising:(a) providing a blank of rolled steel plate of curvedshape with suitable size and thickness for constituting a section of oneof the structural elements, (b) positioning the blank in a two-partpressing and extrusion tool composed of a matrix curved to match thecurved shape of the blank and provided with a matrix opening with arounded transition surface leading into the matrix opening andcorresponding to the desired outside surface of a brace stub to beformed in the blank and, a piston-like pressing tool with a diametersubstantially corresponding to the desired inside diameter of the bracestub to be formed and having a rounded off working end, said pistonbeing arranged to be movable towards and into said matrix opening in thematrix, and (c) forming a brace stub terminating with a brace stubopening from the part of the blank covering the matrix opening by movingthe piston towards the matrix with the blank positioned thereoneffective to press the part of the blank covering the matrix openinginto the matrix opening about the rounded transition surface causing anextrusion deformation of the part of the blank covering the matrixopening to extend the length of the brace stub and to form in the blankthe brace stub having inside and outside surfaces smoothly curved fromthe blank proper towards the brace stub opening with graduallydecreasing wall thickness from the blank proper to the brace stubopening as a result of the extrusion deformation.
 2. Method inaccordance with claim 1, wherein the blank is provided with an initialaperture.
 3. Method in accordance with claim 1, including the furtherstep of forming an initial aperture in the part of the blank coveringthe matrix opening.
 4. Method in accordance with claim 3, wherein theinitial aperture is positioned laterally disposed relative the matrixaxis.
 5. Method in accordance with claim 3, wherein the initial apertureis aligned with the matrix axis.
 6. Method in accordance with claim 1,including the further step of preheating the blank.
 7. Method inaccordance with claim 6, wherein the preheating is from about 800° C. toabout 1000° C.
 8. Method in accordance with claim 1, wherein the pistonis moved toward the matrix in a series of repeated steps.
 9. Method inaccordance with claim 8, wherein the blank is heated between successivesteps.
 10. Method in accordance with claim 1, wherein the brace stub isformed obliquely to the blank proper.
 11. Method in accordance withclaim 1, wherein the brace stub is formed normal to the blank proper.12. Method in accordance with claim 1, wherein two brace stubs areformed from the blank.
 13. Method in accordance with claim 12, whereinthe central axes of the two stubs have different angular orientationswith respect to the blank proper.